The present invention relates to a connection bolt for repeated tightening beyond yield point provided with means making possible to determine the degree of damage by permanent deformations, whereby, the deformations are a criterion for their further serviceability.
At present, connection bolts are tightened up to the threshold of their yield point with the minimum risk of a damage very carefully, but costly, using an electronic device applying the gradient method. Then, the bolts are further applicable, but only for the first time there is a full certainty because usually such a device is available only at the producer premises during the first assembly.
But there are also more available methods of tightening connection bolts up to their yield point. Such methods use a control by the angle of rotation, eventually by the number of impulses of a tightening spindle, but at the cost of certain results scattering, including a random risk that most of them will be over-tightened beyond the yield point what could lead to a partial bolt material damage in such a bolt by permanent deformations.
In spite of that both above mentioned methods of connection bolt tightening guarantee the advantage of using their strength up to 100% during one case of their tightening and reaching of their maximum possible pre-stressing within a narrow scattering range, whereby, they represent the most advanced methods in the art at present, they exhibit an essential costs weakness. The weakness consists in that because of inherent uncertainty in the process of storing information about partial damage of said bolts regarding their tightening beyond their yield point they have to be replaced after each disassembly to meet the quality demands. A paradox is that because of that the connection bolts are a part for one use only. Consequently, this substantially adds to the costs of an overhaul, repair, assembly of disassembled products and service maintenance. The present unavailability of the option of a repeated assembly without a replacement of said bolts prevents a more frequent application of said advanced tightening method in practice.
The prior art comprises also a method of connection bolt damage evaluation which method consists in that the permanent bolt elongation is used as a criterion for the plastic deformation of said bolts. But the problem is that this indicator is not explicit enough and that it is necessary to keep records about the original length of each bolt individually, what is inoperable in an industrial use. Therefore, this method is useful only for a general optical selection of evidently elongated bolts exhibiting a visible contraction of their critical cross section. Similar and even greater disadvantages exhibits also the other method used which method consists in evaluating length elongation only of the threaded part of the bolt shank.
Where a top quality is the priority in the production process, considering the above described uncertainty, in day-to-day practice, preferably, all otherwise highly valuable disassembled connection bolts are discarded whenever there is a suspicion that the yield point was exceeded. But mostly they are still quite well applicable. Thereby, substantial financial losses are caused in spite of that in theory such losses are not necessary from the technical point of view. Considering the above, it is an object of the present invention to provide a method of suitable and explicit system of marking said bolts enabling a reliable indication of the extent of the permanent deformations.
The above mentioned disadvantages of the prior art methods are removed by a connection bolt for repeated tightening beyond yield point which bolt comprises a head and a shank which shank is provided with a thread, according to this invention, whereby said head is provided with at least one indication mark at a fixed defined mutual angle position to at least one indication sign, which sign is provided on the free shank end relative to the axis of bolt rotation. Said indication mark and said indication sign are formed by at least one three-dimensional integral or interrupted groove or projection which groove or projection are formed in the material of said connection bolt. Perpendicular lines dropped from the centres of said indication marks or signs to the bolt rotation axis qxe2x80x94q form reference axes axe2x80x94a or cxe2x80x94c of the respective indication system which reference axes together with said bolt rotation axis qxe2x80x94q define the actual mutual angle position of a head reference plane xcex1 and a bolt shank free end reference plane "khgr". In case that, considering a special symmetric configuration of indication marks or signs, their centre falls directly into the bolt axis qxe2x80x94q, preferably one of other symmetry axes in the system of indication marks or signs, situated in the plane perpendicular to the bolt rotation axis qxe2x80x94q, may be chosen as the reference axis. In case of the connection bolt embodiment which is provided with a guiding tip, the indication sign of the shank free end is formed by at least one flattening or concave surface formed on the guiding nose in radial direction with regard to the connection bolt axis qxe2x80x94q. In case of an embodiment provided with a short cylindrical or conical guiding nose, the indication sign is situated on the face of the free end of the shank and formed by a full-length or interrupted groove or projection. The indication mark and the indication sign, provided already in the production of said connection bolt, determine the angle xcex94xcex8H of permanent torsion of said connection bolt shank by a change of their mutual angle positions, which mutual angle is represented by an angle xcex8H of the reference planes xcex1 and "khgr" after a bolt tightening beyond the yield point, and said indication mark and said indication sign make said angle xcex94xcex8H of permanent torsion visible and measurable. Surface of said indication mark and said indication sign is protected so as the other parts of said connection bolt at least by an equally effective protective layer.
The indication marks and signs serve, either directly as a contact for a device for reading the actual positions in the system of indication marks in the area of said head and of those of the indication signs at the bolt shank free end or in a combination with the gripping surfaces of said head or flattening of said guiding end, as orientation optical marks that make possible to read correctly the size of the respectively defined angle xcex8H of said planes xcex1 and "khgr" in the desired size range. In case, it is necessary to use the entire theoretically possible angle xcex8H size range from 0xc2x0 to 360xc2x0, this requirement can be met e.g. by an intentionally unsymmetric configuration of the figures of said indication marks and signs.